This invention relates to soft contact lenses made of borate crosslinked polyvinyl alcohol.
In general, existing hydrogel soft contact lens materials based on HEMA, also known as hydroxyethyl methacrylate or as ethyleneglycol monomethacrylate, polymers suffer from either weak mechanical strength, low dissolved oxygen permeability or a combination thereof.
Polyvinyl alcohol films and gels have been reported as ophthalmic inserts in the lower conjunctival sac when imbibed with antibiotics such as tetracycline, pilocarpine, atropine and the like. Such materials are either in the form of crosslinked film or as a water soluble viscous solution or gel. See, for example, Y. F. Maichuk, Ophthalmic Drug Inserts, Invest. Ophthalmol., Vol. 14, pages 87-90 (1975); D. W. Lamberts, Solid Delivery Devices, Int. Ophthalmol. Clinic, Vol. 20, No. 3, pages 68-69 (1980) and Y. F. Maichuk, Antibiotik, Vol. 12, No. 4, pages 432-5 (1967). Also polyvinyl alcohol crosslinked with glyoxal has been proposed as a contact lens material. See U.S. Pat. No. 3,408,429. Unfortunately, the acetal and hemiacetal group formation which results from the crosslinking reaction is reversible under mildly acidic conditions, resulting in the potential release of glyoxal from the cross-linked material. Glyoxal is known to be an irritant to skin and mucosa.
In addition, soft contact lenses based upon crosslinked hydrogels generally require either costly lathing and polishing steps, or cumbersome centrifugal casting which is carried out while polymerization and crosslinking is performed in situ during the casting operation.
Further, the use of boric acid and polyvinyl acetate to treat films of polyvinyl alcohol to convert the film into a fogged or hazy state has been described. See Chemical Abstracts, Vol. 66, item 76674f (1967). In contrast, the instant borate-crosslinked polyvinyl alcohol contact lenses possess excellent optical clarity with no degradation of optical clarity being evident.
Crosslinked hydrophilic polymers and hydrogel soft contact lenses made therefrom are known in the art. Such polymers are generally based on HEMA, also known as hydroxyethyl methacrylate or ethyleneglycol monomethacrylate, with one or more optional comonomers as desribed in U.S. Pat. Nos. 2,976,576, 3,841,985 and 3,985,697. Other hydrogels based on N-vinylpyrrolidone copolymers and acrylamide copolymers are disclosed in U.S. Pat. Nos. 3,639,524 and 3,929,741. These prior art hydrogel polymers suffer from either weak mechanical strength, insufficient dissolved oxygen permeability or a combination thereof. Efforts to increase oxygen permeability by increasing the water content of the hydrogels result in a further decline in the mechanical strength of the lens materials to such an extent that they are extremely fragile.
It is an object of the present invention to provide soft contact lenses obviating or substantially reducing the aforementioned drawbacks of the prior art.
It is a further object of the present invention to provide contact lenses comprising polyvinyl alcohol which have been crosslinked with a borate such that the crosslinked lenses are subtantially insoluble in the ocular tear environment, possess a high water content, high dissolved oxygen permeability and superior mechanical strength.
It is a further object of the invention to provide rapid, simple, low cost molding and spin casting processes for the preparation of such lenses.